In a typical gas turbine engine component efficiencies depend increasingly on the clearance between the rotating rotor blades and the annular member, or members, defining the outer gas flowpath. The casing arrangements which form the annular member or members, must ideally have a thermal responsive characteristic matching the growth of the rotor. The rotor growth is characterised by an initial fast centrifugal growth element superimposed on a slower thermal rotor growth element. Furthermore the thermal rotor growth element at the rim of the rotor is a resultant thermal strain between principally the rotor rim and the rotor cob. Due to its direct contact with the main gas stream, the rotor rim has a fast thermal growth rate. The rotor cob however is much slower in its thermal growth rate. Hence the rotor radial displacement profile can be regarded as a multi-rate system responsive to changes in the gas turbine engine conditions.
Many blade tip clearance control arrangements have attempted to match the rotor growth by using several design principles. A first arrangement has used a fast response casing arrangement in which a high thermal expansion material gives a large range of radial displacements. A second arrangement has a slow response casing arrangement in which a low thermal expansion material gives a small range of radial displacements. A third arrangement uses a combination of the first and second arrangements and our UK patents GB1484288, GB1484936 and GB2087979B are examples.
Unfortunately these blade tip clearance control arrangements have failed to produce the ideal solution, as they are essentially single rate thermal systems because small blade tip clearances are achieved at certain engine operating conditions at the expense of other engine operating conditions.